I would figure about 40 gallons per goldfish (150 litters). I would not go much less than that, and would suggest actually stocking with fewer. You will need to add fish ver slowly. Don't add too many fish at once. I would not suggest any bottom feeding fish. Snails will do a good clean up job. Plus goldfish look for food all day and will eat anything they can._________________21 goldfish: 3 orandas, 3 pearlscales, 4 telescopes, 1 lionhead, 1 ranchu, 5 ryukin, and 1 pom. Been keeping and breeding goldfish 16 years.

I agrree, no bottom feeders at all. The goldfish will browse for any missed food and will eat algae also. all sorts of plecos are known to attach goldfish, leaving round marks and lost scales leading to infections.